Theres alot of options for nano fish.
have you checked out the nano fish section at Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
? They have all the smaller fish together in one section.
I would be looking at firefish, gobues, blennies maybe a royal gramma. My current 20g rimless is kinda stocked heavy with 6 fish but I have in there: green clown goby, yellow clown goby, barnacle blenny, two spot blenny, fangtooth blenny and an engineer goby (too big for a 20g, will go into my 125 when he gets bigger).
If I were working around a clown I'd probably get a firefish, a yellow watchman goby, a small blenny and maybe one other fish. I also love my inverts so I would look at fire shrimp and cleaner shrimp too. If those guys are too $$ check out some peppermint shrimp. I also really like my crabs, I have emerald crabs and a porcelain crab in the 20g. I also have 2 peppermints in there.
Hope that helps a bit. Like I said, check out liveaquaria nano section for mor ideas.